A conventionally known device that is disclosed, for instance, by Japanese Patent Laid-Open No. 2002-242713 retards the valve opening timing of the intake valve while the internal combustion engine is cold. When the temperature of the internal combustion engine is within the range of 0 to 50° C., this device selects, for a region retarded from the exhaust top dead center, intake valve opening timing that is retarded from normal valve opening timing.
In a region where the exhaust top dead center is exceeded by a crank angle, the more retarded the intake valve opening timing, the higher the intake flow velocity. Therefore, while the internal combustion engine is cold, the above-mentioned conventional device can provide a higher intake flow velocity than in a normal state. When the intake flow velocity is high, the fuel injected into an intake port is likely to become atomized and unlikely to adhere to the intake port or intake valve.
The more atomized the fuel supplied to the internal combustion engine, the higher the combustion quality. Further, air-fuel ratio control in the internal combustion engine can be accurately exercised when the port wet amount is small. Therefore, the above-mentioned conventional device can enhance the stability of the internal combustion engine and provide increased accuracy in air-fuel ratio control at the time of an internal combustion engine cold start.
The applicant of the present invention has acknowledged that the following documents relate to the present invention including the aforementioned document:
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-242713
[Patent Document 2]
Japanese Patent Laid-Open No. 1994-323168
[Patent Document 2]
Japanese Patent Laid-Open No. 1998-252575